(1) Field Of The Invention
The present invention relates to a method and an apparatus for treating the dielectric used in an electrical discharge machine tool (hereinafter referred to as an "EDM") wherein the dielectric is mixed with powder materials.
(2) Description Of The Background Art
In the EDMing arts, it is well known to perform melt processing or heat treatment on a workpiece made of an electrically conductive material, e.g., metal, by utilizing the high-temperature energy generated in an electrical discharge path. The discharge gap is filled with dielectric having electrical insulation characteristics to obtain the electrical insulation required for discharge. Usually an oil or oil/water mixture is used as the dielectric. Besides obtaining electrical insulation, the dielectric functions to remove chips produced during the machining process and to remove the resultant heat from the machining site, thereby acting as an important factor in maintaining stable EDMing.
Japanese patent application No. 1990-15813, which is owned by the assignee of the present patent application, teaches that mixing the dielectric with certain kinds of materials in the form of powders of approximately 10 to 40 .mu.m in particle diameter, to a mixing density of around 20 g/liter, improves the stability of discharge. Depending on the material, the powder dielectric acts to upgrade mechanical characteristics, e.g., corrosion resistance and wear resistance, of the EDM electrode and the workpiece surface. In addition to the removal of material (e.g., metal) from the workpiece, EDMing allows the metal to be surface-treated. The powder materials used include semiconductor materials, e.g., silicon (Si), zinc oxide (ZnO), zinc dioxide (ZnO.sub.2), silicon dioxide (SiO.sub.2), tungsten carbide (WC) and zirconium boride (ZrB.sub.2), and fine ceramic materials, i.e., carbide and boride.
It has been proposed to machine a workpiece or form a surface layer thereon by mixing such materials in powder form into the dielectric. This proposal would vastly expand the application range of EDM. The problem is to provide for practical implementation of the proposal. Specifically, one goal is to design an apparatus for efficiently treating the dielectric used in EDMing.
One dielectric treating apparatus for an EDM according to the background art is seen in FIG. 23. There an electrode 1 and a workpiece 2 are submerged in a dielectric within an EDMing tank 3. A dielectric tank 4 is connected to tank 3 and receives the dielectric drained from the bottom of tank 3 via valve 10. The drained dielectric may contain roughing chips 5 generated during roughing EDM operations, finishing chips 6 generated during finishing EDM operations and a powdered semiconductor material 7, such as silicon, as mentioned above. A tank 8 contains dielectric, supplied from tank 4 via pump 14 and filter 12, and receives raw powder from powder material supplier 9. A pump 13 operates to supply dielectric from dielectric tank 4 via filter 11 to the EDMing tank 3. The filters 11 and 12 are operative to remove chips from the dielectric pumped by pumps 13 and 14, respectively. The suspended powder is removed as well. A pump 15 supplies dielectric from the tank 8 to a discharge gap between the workpiece 2 and electrode 1 in tank 3.
Operation of the EDM as above constructed can be understood with reference to FIG. 23. In an ordinary roughing process, the workpiece 2 installed in the EDMing tank 3 is EDMed by electrical discharges in a discharge gap formed between the electrode 1 and the workpiece 2. In this process, chips 5 will be produced which will be suspended in the dielectric in the dielectric tank 4. The chips 5 so formed during the machining process will be carried to tank 4 via valve 10 when the dielectric is recirculated. The chip and dielectric suspension is then pumped by pump 13 to filter 11 wherein the chips and any powders are removed and the reclaimed dielectric is circulated back into the EDMing tank 3. When surface treatment, and the like, is to be carried out by finishing machining after roughing, clean dielectric mixed with the powder materials 7 from supplier 9 in tank 8 is pumped by the pump 15 from the tank 8 to the discharge gap formed by the electrode 1 and the workpiece 2. At the same time, dielectric in tank 4, having a suspension of chips and powder materials 7, is pumped from the dielectric tank 4 into the tank 8 via filter 12, in order to remove any chips and powder. The powder materials 7 consumed by EDMing and filtering are replaced by powder from the powder material supplier 9, which is operated to automatically supply the powder materials 7 to the tank 8.
As mentioned above, the powder materials and the chips are treated together in the prior art EDM which does not consider the separation of the chips from the powder materials. In addition, when finishing is performed, especially when accompanied by surface treatment, after a roughing step which produces numerous chips, finishing accuracy is greatly influenced by the numerous chips contained in the finishing dielectric. It is therefore necessary to continuously filter out the chips in the prior art. Finally, since the powder materials are removed from the dielectric with the chips by the filtering process and then discarded, a large quantity of powder materials is consumed, which results in cost disadvantages.
It is, accordingly, an object of the present invention to overcome the disadvantages in the prior art by providing a dielectric treatment apparatus for an EDM, which reduces the consumption of powder materials and allows the same powder materials to be reused over a long period of time.
It is another object of the present invention to filter and store powder materials for reuse with a simple apparatus.
It is a further object of the present invention to automatically detect the deterioration of the powder materials and replace such materials in response to such detection.